Radio receiver



March 22, 1938. p FARNHAM' 2,111,763

RADIO RECEIVER Filed Nov. 25, 1933 2 Sheets-Sheet l March 22, 19386 P. o. FARNHAM 2 1?;L763

' RADIO RECEIVER Filed Nov. 23, 1933 2 Sheets-Sheet 2 Patented Mar. 22, 1938 PATENT OFFICE RADIO RECEIVER Paul -0. Farnham, Boonton, N. J., assignor, by mesne assignments, to Radio Corporation of America, New York, Delaware N. Y., a corporation of Application November 23, 1933, Serial No. 699,460

8 Claims.

This invention relates to radio receivers and particularly to receivers of the remote control type.

It has been proposed to separate the elements of a receiver into a small tuning unit and a larger or main unit which includes the power supply system, the audio frequencysystem and, in some instances, the intermediate frequency amplifier and second detector of a superheterodyne receiver.

-An object of this invention is to provide a radio receiver of the remote control type which includes an economical and efficient transmission line of low impedance for transferring radio frequency currents between spaced units of the receiver. Another object is to provide a remote control receiver of the superheterodyne type and in which radio frequency currents are passed through a low impedance transmission line in a cable, the coupling between the receiver units including a tuned intermediate frequency circuit in each unit and a capacitive coupling between the tuned circuits, the capacitive coupling com prising the inherent capacity between the intermediate frequency lead and the grounded lead of the cable. Further objects of the invention are to improve the construction of radio receivers of the remote control type by reducing the number of leads-required in the cable connecting the units of the receiver and, more particularly, to combine the antenna connection to the remote control tuning unit with one of the leads which supplies an energizing potential to one or more tubes of the control unit These and other objects and advantages of the invention will be apparent from the following specification, when taken with the accompanying drawings, in which:

Fig. l is a perspective View of a complete re-' ceiver assembly embodying the invention;

Fig. 2 is a diagram of the electrical circuits of a receiver; and

Fig. 3 is a simplified diagram of an appropri ate form of antenna system for a two-unit receiver. I r

The invention is applicable to receivers of the type, as illustrated in Fig. 1, which includes a main unit'A and a tuning and control unit B which may be located at any convenient distance, within the limits set by a connecting cable C, from the main unit A. The casing of main unit A may, ofcourse, be of any preferred design, such as that of a lowboy which may be placed against the wall of'the room or adjacent a chair where it serves as an end table. The

control unit B may be much smaller than the main unit and preferably takes the form of a small box having a hinged lid for concealing the usual controls and dial when the receiver is not in use.

The receiver elements within the control box B may include a carrier wave amplifier tube I a combined oscillator-detector tube 2, and the tuned circuits up to and including one tuned intermediate frequency circuit. The larger unit includes additional tuned intermediate frequency amplifier circuits and an associated amplifier tube 3, a second detector and audio amplifier 4, a reproducer 5, a gain control tube 6 and associated elements for automatically regulating the carrier frequency gain, and a source of power such as the power pack I.

The antenna 8 is preferably a part of the main unit and is coupled to the tuned input circuit 9 by coil Hi of relatively few turns, the antenna 8 being connected. to coil I0 through cable C, as is described in detail hereinafter.

The usual transformer coupling may be employed between tubes 1 and 2, the secondary being shunted by a tuning condenser to provide a tuned carrier wave circuit ll. Tube 2 is of the multiple electrode type and includes inner grids G1, G2 which cooperate with cathode K to function as an oscillator, and additional grids and a plate which cooperate with the cathode to function as a shielded tetrode amplifier. The oscillator inductance I2 is connected to grid G1 and, through the usual series capacity E3, to ground. The grid leak l4 may be connected across capacity' I3 and the oscillator tuning condenser I5 is mechanically connected to the tuning condensers of the carrier wave circuits 9, II, as indicated by the broken line Hi. The plate coil of the oscillator is connected to grid G2 and, through a carrier frequency choke H, to the lead b of the cable C.

The high potential terminal of the tuned circult II is connected to grid Go, and the intermediate frequency inductance i8 is connected between plate P and the cable lead b, the shunt condenser 19 for tuning inductance it being connected between plate P and ground. Lead b preferably constitutes the only path for supp-lying direct current potentials to the screens and anodes of tubes I, 2, suitable resistances 20 being included in the screen grid circuits for reducing the screen potentials to their desired values. The lead I) is enclosed in a flexible metallic sheath which constitutes a shield and which, preferably, formsthe ground lead a of the cable.

The heaters of tubes I and 2 are preferably connected in series and are supplied with current by leads c, d of the cable. As best shown in Fig. 3, the heater circuit is supplied with alternating current of appropriate voltage from the secondary winding 2| of the power transformer, the winding having a center tap which is connected to ground through the coil 22 that is coupled to inductance 23 in the antenna circuit. The carrier wave voltages though introduced into the heater circuit are transferred to the coil IU of the control unit by connecting the coil I6 between ground and a point 24 which represents the junction of the twoserially connected heaters H. The coils 22, 23 constitute a step-down transformer while coils I0, 9 constitute a corresponding step-up transformer, and the heater circuit thus provides a low impedance transmission line for introducing the received carrier waves into the control unit. The usual dial light 21 may be connected across one of the heaters H.

The automatic gain control voltage developed by tube 6 of the main unit is applied to the control grids of tubes I and 2 through the cable lead e, the connections including the usual de-coupling resistances 28. The cathodes K of tubes I and 2 are joined and connected to ground through a biasing resistance 29 which provides a control grid bias appropriate for maximum amplification. With increasing signal strength, however, the amplification of tube I and translation gain of tube 2 are automatically reduced to maintain an approximately constant output from tube 2.

The unit B also includes a manual control for regulating the room level or output from reproducer 5, the control comprising an adjustable resistance 30 which is connected between ground and leaf 1 of the cable.

In the main unit A, the lead a of the cable is grounded, and the inductance 3| and condenser 32 of a tuned intermediate frequency circuit are serially connected between lead I) and ground. The lead I) is also connected to ground through a condenser 33 which, in parallel with the inherent capacity 33, Fig. 3, between leads aand b, provides a capacitive coupling between the two tuned circuits I8, I9 and 3|, 32.

The magnitude of the total coupling capacity is preferably such that the circuits have a narrow band-pass characteristic. It will be apparent that the tuned intermediate frequency circuits are coupled through a low impedance circuit and that, as best shown in Fig. 3, the low impedance coupling is provided by the essential elements of the tuned circuits, thus avoiding the use of the complementary transformers commonly employed in telephone practice to secure low impedance transmission lines.

The tuned circuit 3|, 32 is loosely coupled magnetically to tuned circuit inductance 34 and condenser 35 to provide further selectivity in the intermediate frequency amplifier. The voltage developed across the last tuned circuit is impressed on the amplifier tube 3 which, as illustrated, works directly intoithe second detector. Recent tube developments have made it possible to obtain sufiicient amplification with a single intermediate frequency stage, but it is to be understood that the essential feature of the intermediate frequency amplifier resides in the capacitive coupling of tuned circuits which may be located in separated cabinets or units, and that more than one intermediate frequency amplifier tube may be used.

If more than one intermediate frequency amplifier tube is employed, such additional tube or tubes and their associated circuits will be introduced at the point x, it being understood that the illustrated tube 3 is preferably the amplifier tube which works into the second detector.

The cathode of tube 3 is connected to the manual control leaf f of cable C through a bias resistor 36 which limits the minimum negative bias that may be placed on tube 3 by adjustment of resistance 30 to its minimum or zero value. The output or room level of the receiver is thus controlled by the variation of the direct current bias on the last intermediate frequency amplifier tube, and this method of control avoids the difiiculties encountered and expensive constructions required when audio frequency voltages are transmitted through a cable system.

The automatic control tube 6 is connected across the input circuit of tube 3 through a small coupling condenser 31. Tube 6 is preferably of the type including an anode A1 in addition to a conventional pentode structure. The intermediate frequency voltage is impressed upon control grid G1 and, after amplification, is transferred to the diode rectifier elements A1 and cathode through the coupling transformer 38, the direct current output voltage being developed across resistance 39. The negative direct current potential thus developed is applied through the alternating current filter, comprising resistance 40 and condenser 4 I, to the gain control lead e of the cable.

The direct current drop across the cathode resistance 42 of tube 6 makes the cathode positive with respect to anode A, and therefore renders the automatic control system inoperative for relatively weak signals. Only a portion of this high negative bias is applied to the amplifier section of the tube by the resistance 43 which connects control grid G1 to an intermediate point on resistance 42.

The usual bypassing condensers are illustrated in the several circuits-but such elements will not be listed and separately mentioned as the necessity for and methods of bypassing are well known. It is to be understood that the power supply may be from batteries or other direct current source, and that the legends +90 volts, etc., indicate Values appropriate for one particular set of tubes. The voltages to be employed in any given case will be determined by the usual design requirements since the invention does not impose any additional limitations in this respect.

Particular attention is directed to the fact that the alternating currents transmitted through the cable are of intermediate frequency, for example of 175 kilocycles, and cycles in the usual case of a power supply from the usual house lighting system. The low impedance coupling employed in the intermediate frequency circuit and the shielding of lead b prevent any interference or noisy operation from 60 cycle or stray alternating currents of other frequencies. The design of the manual control of the room level or reproduction makes it possible to eliminate the special constructions required for good fidelity when audio frequency currents are transmitted through a cable.

The quality of the reproduced audio frequency is not affected by the mechanical separation of the receiver into two units and is identical with that obtained when circuits of the same design are housed in a single unit. While the full advantages of the invention as a substitute for the prior types of remote control receivers are obtalned when the size of the tuning unit B is made as small as possible, the invention is not restricted to any given size of this unit and, where size is not an object, the design of the carrier frequency amplifier, first detectorand local oscillator may be varied at will.

While the preferred form of the invention has been illustrated, it will be understood that the same method of obtaining a low impedance transmission line between separated radio frequency circuitsmay be employed when carrier wave frequencies are to be transmitted through a cable connection. The antenna input to the control unit may be passed through any one of the leads to the control unit but, preferably, the selected lead is one which is not traversed by the intermediate frequency.

I claim:

1. In a radio receiver of the remote control type, a control unit comprising a tuned radio frequency amplifier terminating in a fixedly tuned circuit, a main unit including a circuit tuned to the same frequency as that of said first tuned circuit, a pair of leads in a cable connecting the terminals of said tuned circuits, a capacitive reactance common to said two tuned circuits constituting the sole coupling impedance between said two tuned circuits, said capacitive reactance including the inherent capacitive coupling between said leads and the reactance having a magnitude sufiicient to impart a narrow band-pass characteristic to the coupled tuned circuits.

2. A receiver as claimed in claim 1, wherein each of said tuned circuits. includes a parallel tuned circuit comprising an inductance and a condenser, each condenser having one terminal connected to one terminal of its inductance and the opposite terminal of each condenser being connected to one of said cable leads, the second terminal of each inductance being connected to the second cable lead.

3. In a'radio receiver of the superheterodyne type, the combination with a control unit comprising a tuned carrier frequency circuit, an oscillator and a first detector, a parallel tuned inter- -mediate frequency circuit comprising an inductance and a condenser having common terminals connected to the plate of said detector, the other terminal of the condenser being grounded, of a main unit including a parallel tuned intermediate frequency circuit comprising an inductance and a condenser having a common high potential terminal, the second terminal of the condenser being grounded, of a cable connecting said units, said cable including a grounded lead and a second lead directly connecting the second terminals of the said inductances, whereby capacity between said leads provides a capacity coupling between said circuits.

4. A receiver as claimed in claim 3, in combination with a condenser in one of said units and having its opposite sides connected directly to said respective leads to supplement the inherent capacity between said leads.

5. In a radio receiver of the remote control type, a tuning unit including a tuned carrier frequency circuit and a vacuum tube amplifier, a main unit including a power supply system and an audio frequency amplifier working into a reproducer, a cable connecting said units and including a plurality of leads for transferring energizing potentials from said power supply system to the said tube of the tuning unit, a transformer secondary connected across two of said leads, a carrier frequency transformer in said main unit, said transformer having a secondary effectively connected to the midpoint of said first named secondary and a primary adapted to receive a connection from an antenna structure, and means in said tuning unit coupling one of said leads to the said tuned carrier frequency circuit.

6. A radio receiver of the remote control type,

said receiver comprising a tuning unit including a vacuum tube having 'an input circuit tunable over a range of carrier frequencies, a main unit including a power supply system and a plurality of cascaded tubes forming a radio frequency amplifier and an audio frequency system, a cable having a pair of leads connected to the output of said tuning unit for transferring radio frequency currents from said tuning unit to said main unit, additional leads in said cable for transferring energizing potentials from said main unit to said vacuum tube of the tuning unit, and means including said additional leads. arranged for transferring received carrier wave energy through said cable to said tunable input circuit said means comprising a transformer in said main circuit, said transformer being provided with a primary winding adapted to receive a connection from an antenna and a secondary in series with one of the'additional leads, and a coil in said tuning unit, said coil being coupled to said tunable circuit and in series with the said one additional lead.

7. A radio receiver as claimed in claim 6, wherein said power supply system includes a winding for supplying current to the cathode-heating circuit of said first named tube, and a pair of said additional leads connect said current-supply winding to the cathode-heating circuit of said first named tube, said current supply winding having a center tap, a transformer having a primary adapted to be connected between an antenna structure and ground, the transformer secondary being connected between said center tap and ground, and a coil in said tuning unit coupled to said tunable circuit and connected between ground and the cathode-heating circuit of said tube.

8. Aradio receiver as claimed in claim 6, wherein the said radio frequency amplifier of said main unit amplifies at an intermediate frequency, and said tuning unit includes means for amplifying received signals at carrier frequency and for beating the amplified carrier frequency signals to an intermediate frequency.

PAUL O. FARNHAM. 

